1. Field of the Invention
This is a method for processing seismic signals to improve the resolution of desired signals in the presence of unwanted signals.
2. Discussion of Relevant Art
The art of seismic exploration for natural resources is very well known. Nevertheless, a brief tutorial follows.
An acoustic source of any well-known type is caused to radiate a wavefield (fire a shot) into a body of water from a source location at or near the surface. The wavefield may be radiated by an impulsive device such as air gun, by a chirp-signal generator or by an implosive device. The acoustic radiator may be a single point-source or an array of point sources arranged in a desired pattern. Hereinafter for brevity, we will simply use the term "source". Activation of a source is a "shot".
The radiated wavefield propagates in all directions, insonifying the subsurface earth layers whence the wavefield is reflected back to the surface of the earth where the reflected wavefield is detected by one or more acoustic receivers. The acoustic receivers may be of any type having a capability for converting mechanical wavefields to electrical signals. Suitable receivers for deep water marine use include pressure sensors (hydrophones) that are omni-directionally responsive to acoustic stimuli. The term "receiver" as used herein includes a single instrument or a group of several electrically-interconnected receivers arranged in a desired geometric pattern at or near the surface of the earth.
In deep-water marine operations, the receivers are mounted in a long streamer cable and towed behind a ship along a line of survey. The electrical signals from the receiver(s) are delivered through data transmission means installed in the cable to signal conditioning and archival data storage channels, one channel per receiver. The data transmission means may be electrical-wireline, optical, or ethereal in nature. Acoustic data-transmission channels are also known.
The electrical signals representative of the arrival times of reflected wavefields at the respective receivers are digitized and recorded on reproducible, computer-readable recording media such as, but not limited to, photographic time scale recordings, magnetic tapes, floppy disks, CD-ROMs or any other archival data-recording device now known or as yet unknown.
The recorded data may be sent to a processing center where the data are introduced to a suitable general purpose computer which is programmed to process the seismic data, thereby to construct a model of the earth's subsurface. Programs in the computer include formulations and algorithms that exist for the sole purpose of operating on the digitized seismic data signals to convert those signals into a different state such as a desired visual model of a volume of the earth. The resulting model is used by geologists in recovering valuable natural resources such as oil, gas or other useful minerals for the benefit of humankind. That is, data-processing algorithms exist to process the gathered seismic signals into a useful, human-interpretable format; the data are not gathered simply to provide a solution to some naked algorithm.
Geophysical studies may be one- or multi-dimensional. In a two-dimensional survey by way of example but not by way of limitation, a source and an array including a plurality, numbering in the hundreds or thousands, of spaced-apart receivers are towed along a line of survey as previously explained, one receiver or receiver group per data channel. The receivers, preferably separated from one another by an interval such as 25 meters, are distributed along the line of survey at increasingly greater offset distances from the source. The range in offsets from a source, which is usually towed close to the towing ship, ranges from 200 meters to the nearest receiver to as much as 30 kilometers to the most distant receiver. The source is usually fired at timed intervals such that, at the usual ship velocity of six knots, the physical locations of the source, at the actual times of successive firings, are spaced-apart by some multiple of the receiver spacing such as 100 meters.
Unwanted noise contaminates the desired seismic signals. The term noise is defined as any unwanted seismic signal. Noise may be random or coherent. Random noise may be filtered out by use of various well-known forms of stacking such as common midpoint(CMP) gathers. However coherent noises, such as multiple reflections, that often occupy the same domain in time and space as the desired signals, requires a more sophisticated filtering process.
One such process may be implemented by use of the well-known Radon transform. For example, Beylkin, in U.S. Pat. No. 4,760,563, which issued Jan. 9, 1986, discloses a method and system for discrete transformation of measurements such as seismic data into and out of tau-p space which is both exact and practical in terms of processing time. The measurements can be filtered or otherwise processed in tau-p space in ways which are not practical or possible in their original space. Since the transformations into and out of tau-p space are exact, the filtered and transformed measurements are free of certain errors and distortions that perturb known approximate transforms which can be performed in a reasonable time. When the transformation process is carried out in frequency space, it is done frequency-by-frequency, and when it is carried out in the spatial domain, it can utilize a transformation matrix having a block circulant structure. In each case, the transformation process and matrix have a structure which substantially reduces storage and processing requirements as compared with other known prior art.
D. Hampson, in a paper delivered at the annual International SEG convention in 1987, entitled Inverse Velocity Stacking for Multiple Elimination, models and attenuates long period multiple reflections by incorporating a parabolic modeling scheme in place of a hyperbolic modeling algorithm.
Beylkin offers a novel approach but the requirement for a transform first to tau-p space and later a second transform to (f-k.sub.p) space and back render that process greedy of computer time. Hampson offers a more economical process in terms of computing time. However, there remains a need to provide an even simpler method for resolving interfering events and for scavenging unwanted noise, both coherent and random.